This invention relates to sonar systems and more particularly to improvements in sonar beamforming through signal processing.
Beam formation is a well known process by which the directionality of response of an acoustic array of transducers is controlled. The individual transducer elements of an array may be arranged with any spatial distribution, subject only to a constraint on the separation of adjacent elements. A beam pattern is a plot of the response of the array as a function of angle, and shows the direction of maximum response, angular width of the major response lobe (beamwidth), and position and level of minor response lobes. In signal processing terms, beam formation is called "spatial processing," and concerns the method and implementation by which beams are formed.
Conventional beamforming or spatial processing is accomplished through a linear weighted combination of the element's outputs. That is, the beamformer output E(.theta.,t) is ##EQU1## WHERE E.sub.J *(.theta.,T) IS THE COMPLEX VOLTAGE DEVELOPED BY THE JTH ELEMENT WHEN THE SIGNAL ARRIVES FROM THE ANGLE .theta. AND AT TIME T, AND A.sub.j * is the complex weighting coefficient which accomplishes beam formation.
Normally, the A.sub.j * are complex, and the processor is termed a "time delay beamformer." If the A.sub.j * are truncated to the first revolution (A.sub.j *=A.sub.j exp (i.phi.) where i=.sqroot.-1 and -.pi.&lt;.phi.&lt;.pi.), then the processor is termed a "phase shift beamformer."
The conventional beam formation process is implemented by use of the components shown in FIG. 1. Sensor or elements 1 through M are allowed to be distributed in space in almost any configuration subject to a spacing constraint. Depending upon the chosen beam formation direction (the major response axis, MRA), the .OMEGA.'s are chosen to phase shift or time delay the individual acquired signal so that all signals at the input to the adder appear to be in phase or in time coherence. The scaled or weighted sum of such signals is obviously maximum for targets on the MRA and will decrease when target signals arrive at the array at angles other than the MRA.
For high resolution sonar arrays (small beamwidth), a large number of sensors is required. Small beamwidth requires the array to have a large lateral dimension in terms of the wavelength of sound in water, and the spacing constraint is usually about two elements per wavelength. Furthermore, when a single beam is scanned throughout the required look sector, an unacceptably low data rate is usually attained. A solution is to process the signal with many preformed beams, each covering its fixed portion of the required look sector. To construct such a preformed beam processor requires that the equipment illustrated in FIG. 1 be replicated once for each preformed beam.
The .OMEGA.'s in FIG. 1 for a time delay beamformer are implemented with electronic delay lines. There are certain drawbacks to this system, however. Delay lines with the required accuracy are rather large and expensive. In addition, because the required accuracy of delay increases for the higher resolution systems, a definite upper limit to resolution exists with delay line beamformers.
If an array of sensors distributed in space were combined as shown in FIG. 1, all the contributing sensors would appear to lie in a plane normal to the MRA. This is the so-called "phasing or time delaying to a plane." Such processing precludes temporal processing on the array, such as replica correlation, signal averaging, or spectrum filtering. Temporal processing is therefore accomplished electronically after the spatial processing.